Measuring Earthquakes

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Vocabulary

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Focus - The point beneath Earth's surface where rock breaks under stress and causes an earthquake.
Epicenter - The point on Earth's surface directly above an earthquake's focus.
P waves - A type of seismic wave that compresses and expands the ground.
S waves - A type of seismic wave that moves the ground up and down or side to side.
Surface waves - A type of seismic wave that forms when P waves and S waves reach Earth's surface.
Seismograph - A device that records ground movements caused by seismic waves as they move through Earth.
Magnitude - The measurement of an earthquake's strength based on seismic waves and movement along faults.
Mercalli scale - A scale that rates earthquakes according to their intensity and how much damage they cause.
Richter scale - A scale that rates seismic waves as measured by a particular type of mechanical seismograph.
Moment magnitude scale - A scale that rates earthquakes by estimating the total energy released by an earthquake.

Outline

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Seismic Waves

• During an earthquake, seismic waves race out from the focus in all directions.
  • Seismic Waves are vibrations that travel through Earth carrying the energy released during an earthquake.
    • Seismic waves carries the energy of an earthquake away from the focus, through the Earth's interior, and across the surface.
• There are three categories of seismic waves: P waves, S waves, and Surface waves.
  • Primary Waves (P waves)
    • P waves are the first wave to arrive .
    • P waves are earthquake waves that compress and expand the ground.
    • P waves cause buildings to contract and expand.
  • Secondary Waves(S waves)
    • After P waves comes S waves.
    • S waves are earthquake waves that vibrate from side to side as well up and down
    • They shake the ground back and forth.
      • When S waves reach the surface, the shake structures and buildings violently.
      • S waves cannot move through liquids like P waves can.
  • Surface Waves
    • When S and P waves reach the surface, some of the waves are transformed into a seismic wave called surface waves.
    • Surface Waves move more slowly than P and S waves, but the produce more severe ground movements.

Detecting Seismic Waves

• To recurd and measure the vibrations of seismic waves, geologists use instruments called seismographs.
  • Untill recently, scientists use mechanical seismographs.
  • Today, geologists use electronic seismographs.

Measuring Earthquakes

• There are three types of ways to measure earthquakes: The Mercalli Scale, the Richter Scale, and the Moment Magnitude Scale.
  • The Mercalli Scale
    • Early in the 20th century, the Mercalli scale was developed th rate earthquakes according to their intensity.
      • The Mercalli Scale is not a precise measurement.
  • The Richter Scale
    • The Richter Scale is a rating of the size of a seismic wave mueasured by a particular type of mechanical seismograph.
    • The Richter scale was developed in the 1930's.
      • Geologists use this scale all over the world for about 50 years.
      • The Richter Scale provides accurate measurements for small, nearby earthquakes.
      • The scale does not work well for large or distant earthquakes.
  • The Moment Magnitude Scale
    • Today, geologists still use this scale.
      • This scale can be used to measure /rate earthquakes of all sizes, near or far.
      • Earthquakes with a magnitude below 5.0 on the scale can cause small and little damage.

Locating the Epicenter

• Geologists use seismic waves to locate an earthquake's epicenter.

• Seismic waves travel at different speeds.
  • P waves comes first
  • S waves comes second.(close behind)
    • The farther away an earthquake is, the greater the time between the arrival of the P and S waves.
      • Geologists then draw 3 circles using data from different seismographs set up at stations all over the world.

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